Cobalt chemistry for smooth topology

ABSTRACT

An electroplated cobalt deposit and a method of electrodepositing cobalt on a surface to produce a level deposit across the surface of the substrate. The cobalt electrolyte contains (1) a source of cobalt ions; (2) boric acid; (3) a pH adjuster; and (4) an organic additive, which contains a suppressor. The electroplated cobalt deposit exhibits a level surface such that the thickness difference across substantially the entire surface of the substrate of less than about 200 nm.

FIELD OF THE INVENTION

The compositions and processes described herein generally relate toelectrolytic deposition chemistry and methods for depositing cobalt andcobalt alloys. These compositions and methods are used for cobalt-basedmetallization of interconnect features in semiconductor substrates. Ascreening method for identifying suitable bath additives is alsodescribed.

BACKGROUND OF THE INVENTION

In damascene processing, electrical interconnects are formed in anintegrated circuit substrate by metal-filling of interconnect featuressuch as vias and trenches, formed in the substrate. Copper is apreferred conductor for electronic circuits. Unfortunately, when copperis deposited on silicon substrates it can diffuse rapidly into both thesubstrate and dielectric films (such as SiO₂ or low k dielectrics).Copper also has a tendency to migrate from one location to another whenelectrical current passes through interconnect features in service,creating voids and hillocks. Copper can also diffuse into a device layerthat is built on top of a substrate in a multilayer device application.Such diffusion can be detrimental to the device because it can damage anadjacent interconnect line and/or cause electrical leakage between twointerconnects. Electrical leakage can result in an electrical short andthe corresponding diffusion out of the interconnect feature can disruptelectrical flow.

Along with the reduction in size and desired increase in the performanceof electronic devices, the demand for defect free and low resistivityinterconnects in the electronic packaging industry has become critical.As the density of an integrated circuit within a microelectronic devicecontinues to increase with each generation or node, interconnects becomesmaller and their aspect ratios generally increase. The build-upprocess, using barrier and seed layers, prior to damascene copperelectroplating, now suffers from disadvantages that are becoming moreevident as the demand for higher aspect ratio features and higherquality electronic devices increases. As a result, there is a need formore suitable plating chemistry to enable defect free metallization.

When submicron vias and trenches are filled by electrolytic depositionof copper, it is generally necessary to first deposit a barrier layer onthe walls of the cavity to prevent the diffusion and electromigration ofcopper into the surrounding silicon or dielectric structure. In order toestablish a cathode for the electrodeposition, a seed layer is depositedover the barrier layer. Barrier and seed layers can be very thin,especially where the electroplating solution contains a properformulation of accelerators, suppressors, and levelers. However, as thedensity of electronic circuitry continues to increase, and the entrydimensions of vias and trenches become ever smaller, even very thinbarrier and seed layers progressively occupy higher fractions of theentry dimensions. As the apertures reach dimensions below 50 nm,especially less than 40 nm, 30 nm, 20 nm, or even less than 10 nm (8 or9 nm), it becomes increasingly difficult to fill the cavity with acopper deposit that is entirely free of voids and seams. The mostadvanced features have bottom widths of only 2-3 urn, a middle width ofabout 4 nm, and a depth of 100 to 200 nm, translating to an aspect ratioof between about 25:1 and about 50:1.

Electrolytic deposition of cobalt is performed in a variety ofapplications in the manufacture of microelectronic devices. For example,cobalt is used in capping of damascene copper metallization employed toform electrical interconnects in integrated circuit substrates. However,because cobalt deposits have higher resistivity, such processes have notpreviously offered a satisfactory alternative to electrodeposition ofcopper in filling vias or trenches to provide the primary interconnectstructures. In a typical semiconductor process, a chemical-mechanicalpolishing/planarization (CMP) step follows electrodeposition in order topolish off overplated deposit or overburden. Rough or uneven surfacescan cause defects from CMP, so it is critical to have a smooth topologyfor overburden. In addition, rough or uneven surfaces can be especiallyproblematic due to the difference in overburden (OB) thickness betweenfeature and non-feature areas, which is normally expected due tomomentum plating. This difference in overburden thickness can be furtherenhanced with the density of features.

U.S. Pat. Pub. No. 2016/0273117 to Doubina, the subject matter of whichis herein incorporated by reference in its entirety, describes methodsand apparatus for electroplating cobalt on a substrate in whichelectroplating may occur through a bottom-up mechanism. Doubina usesvarious plating additives, including particular combinations ofaccelerators and suppressors and particular conductivity to achievedesired plating results. However, Doubina does not mention thedesirability of minimizing changes in overburden thickness betweenfeature areas and non-feature areas of the substrate nor does Doubinadescribes any method of evaluating the effectiveness of bath additivesin a cobalt electrolyte to achieve a desired result.

U.S. Pat. Pub. No. 2009/0188805 to Moffat et al., the subject matter ofwhich is herein incorporated by reference in its entirety, describes amethod of electrodepositing at least one ferromagnetic material, whichmay be nickel, cobalt, or iron, into a three dimensional pattern withina substrate. Moffat describes controlling the potential between anelectrode and a counter electrode, but does not describe any way toevaluate the effectiveness of bath additives.

U.S. Pat. Pub. No. 2019/0010624 to Commander et al., the subject matterof which is herein incorporated by reference in its entirety, describescompositions and methods of using such compositions forelectrodepositing cobalt onto semiconductor base structures comprisingsubmicron-sized electrical interconnect features. The interconnectfeatures are metallized by contacting the semiconductor base structurewith a cobalt electrolytic composition and an electrical current issupplied to the electrolytic composition to deposit cobalt onto the basestructure and fill the submicron-sized features with cobalt.

U.S. Pat. Pub. No. 2019/0093248 to Josell et al., the subject matter ofwhich is herein incorporated by reference in its entirety, describessuperconformally filling a recessed feature with superconformallydeposited gold. However, the process in Josell is directed to theso-called “coinage metals” of gold, silver, and copper, which use verydifferent additives than a cobalt plating bath. While Josell describeselectrochemical measurements, Josell does not mention the desirabilityof minimizing changes in overburden thickness between feature areas andnon-feature areas of the substrate, nor does Josell contemplate how toevaluate the effectiveness of various bath additives.

Thus, it can be seen that it would be desirable to provide a method ofevaluating bath additives for use in a cobalt electroplating baths toproduce a desirable result.

To that end, the inventors of the present invention have surprisinglydiscovered that cyclic voltammetry can be used to analyze and/or screenadditives for use in cobalt electroplating compositions and to determineadditives that are capable of producing a suitable/desired result.

Cyclic Voltammetry can be used to study qualitative information aboutelectrochemical processes under various conditions, and to determine theelectron stoichiometry of a system, the diffusion coefficient of ananalyte, and the formal reduction potential, which can be used as anidentification tool. In addition, CV can be used to determine theconcentration of an unknown solution by generating a calibration curveof current vs. concentration.

Cyclic voltammetry (CV) is a potentiodynamic electrochemical measurementthat can be used to probe reactions involving electron transfers. In acyclic voltammetry experiment, the working electrode potential is rampedlinearly versus time in cyclic phases. During the initial forward scan(from t₀ to t₁) an increasingly reducing potential is applied. Thus, thecathodic current will, at least initially, increase over this timeperiod assuming that there are reducible analytes in the system. At somepoint after the reduction potential of the analyte is reached, thecathodic current will decrease as the concentration of reducible analyteis depleted. If the redox couple is reversible then during the reversescan (from t₁ to t₂) the reduced analyte will start to be re-oxidized,giving rise to a current of reverse polarity (anodic current) to before.The more reversible the redox couple is, the more similar the oxidationpeak will be in shape to the reduction peak. Thus, cyclic voltammetrycan provide information about redox potentials and electrochemicalreaction rates.

The rate of voltage change over time during each of these phases isknown as the experiment's scan rate (V/s). The potential is measuredbetween the working electrode and the reference electrode, while currentis measured between the working electrode and the counter electrode.These data are plotted as current (i) versus applied potential (E, oftenreferred to as just “potential”).

After the set potential is reached in a CV experiment, the workingelectrode's potential is ramped in the opposite direction to return tothe initial potential. These cycles of ramps in potential may berepeated as many times as needed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cobalt depositthat exhibits smooth topology for overburden.

It is another object of the present invention to minimize the change inoverburden thickness between feature and non-feature areas.

It is another object of the present invention to provide a void-freecobalt deposit.

It is still another object of the present invention to minimizeimpurities in the cobalt electrodeposit.

It is still another object of the present invention to provide ascreening method to evaluation the effectiveness of various bathadditives for use in a cobalt electroplating bath.

To that end, in one embodiment, the present invention relates generallyto an electroplated cobalt deposit on a surface of a substrate, whereinthe electroplated cobalt deposit exhibits a thickness difference acrosssubstantially the entire surface of the substrate of less than about 200nm.

In another embodiment, the present invention relates generally to amethod of electrodepositing cobalt onto a substrate comprising recessedfeatures and non-feature areas, wherein the cobalt deposit exhibits athickness difference across substantially the entire surface of thesubstrate of less than about 200 nm the method comprising:

-   -   a) receiving the substrate in an electroplating chamber;    -   b) immersing the substrate into a cobalt electrolyte, the cobalt        electrolyte comprising:        -   (1) a source of cobalt ions;        -   (2) boric acid;        -   (3) a pH adjuster; and        -   (4) an organic additive, wherein the organic additive            comprises a suppressor; and    -   c) electroplating cobalt into the features and onto the onto the        non-feature areas for a period of time and under conditions to        achieve a level, seam-free, bottom-up fill in the recessed        features and plating on the non-feature areas.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a Cyclic Voltammetry curves of Comparative Example 1,Example 2, and Example in accordance with the present invention.

FIG. 2 depicts a photograph of a cobalt deposit as set forth inComparative Example 1.

FIG. 3 depicts a photograph of a cobalt deposit as set forth in Example2.

FIG. 4 depicts a photograph of a cobalt deposit as set forth in Example3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

“A,” “an,” and “the” as used herein refer to both singular and pluralreferents unless the context clearly dictates otherwise.

As used herein, the term “about” refers to a measurable value such as aparameter, an amount, a temporal duration, and the like and is meant toinclude variations of +/−15% or less, preferably variations of +1-10% orless, more preferably variations of +/−5% or less, even more preferablyvariations of +/−1% or less, and still more preferably variations of+/−0.1% or less of and from the particularly recited value, in so far assuch variations are appropriate to perform in the invention describedherein. Furthermore, it is also to be understood that the value to whichthe modifier “about” refers is itself specifically disclosed herein.

As used herein, spatially relative terms, such as “beneath”, “below”,“lower”, “above”, “upper” and the like, are used for ease of descriptionto describe one element or feature's relationship to another element(s)or feature(s) as illustrated in the figures. It is further understoodthat the terms “front” and “back” are not intended to be limiting andare intended to be interchangeable where appropriate.

As used herein, the terms “comprises” and/or “comprising,” specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

As used herein the term “substantially-free” or “essentially-free” ifnot otherwise defined herein for a particular element or compound meansthat a given element or compound is not detectable by ordinaryanalytical means that are well known to those skilled in the art ofmetal plating for bath analysis. Such methods typically include atomicabsorption spectrometry, titration, UV-Vis analysis, secondary ion massspectrometry, and other commonly available analytically methods.

In one embodiment, the present invention relates generally to anelectroplated cobalt deposit on a surface of a substrate, wherein theelectroplated cobalt deposit exhibits a thickness difference acrosssubstantially the entire surface of the substrate of less than about 200nm.

In another embodiment, the present invention relates generally to amethod of electrodepositing cobalt onto a substrate comprising recessedfeatures and non-feature areas, wherein the cobalt deposit exhibits athickness difference across substantially the entire surface of thesubstrate of less than about 200 nm the method comprising:

-   -   a) receiving the substrate in an electroplating chamber;    -   b) immersing the substrate into a cobalt electrolyte, the cobalt        electrolyte comprising:        -   (1) a source of cobalt ions;        -   (2) boric acid;        -   (3) a pH adjuster; and        -   (4) an organic additive, wherein the organic additive            comprises a suppressor; and        -   (5) optionally, one or more additional bath additives; and    -   c) electroplating cobalt into the features and onto the onto the        non-feature areas for a period of time and under conditions to        achieve a level, seam-free, bottom-up fill in the recessed        features and plating on the non-feature areas.

The source of cobalt ions may be selected from the group consisting ofcobalt sulfate, cobalt chloride, chloride sulfamate, and combinations ofone or more of the foregoing. In one embodiment, the source of cobaltions comprises cobalt sulfate heptahydrate.

In a preferred embodiment, the cobalt electroplating composition is atleast substantially free of other metal ions, meaning that the contentof other metal ions is less than about 1% by weight, more preferablyless than 0.1% by weight, most preferably less than about 0.01% byweight. Most preferably, the cobalt electroplating composition is freeof any metal ions except cobalt ions.

Based thereon, the cobalt electroplating composition is preferably atleast substantially free of copper ions. Although very minor coppercontamination may be difficult to avoid, it is particularly preferredthat the copper ion content of the bath is no more than 20 ppb, e.g., inthe range of 0.1 ppb to 20 ppb. In compositions defined herein,“substantially free of copper ions” means that there are less than 20ppb copper ions in solution.

The cobalt ion concentration in the electroplating solution is typicallyin the range of about 1 and about 50 g/L, preferably about 2 to about 25g/L, more preferably about 2 to about 10 g/L, more preferably about 2 toabout 5 g/L.

The electrolytic cobalt composition, also optionally, but preferablycomprises a buffer to stabilize the pH. One preferred buffer is boricacid (H₃BO₃), which may be incorporated into the composition in aconcentration between about 5 and about 50 g/L, preferably between about15 and about 40 g/L. The pH of the composition is preferably maintainedin the range of about 0.5 to about 8. In one embodiment, the pH of thecobalt composition is preferably maintained in a range of less than 5,more preferably in a range of about 1 to about 5, more preferably in arange of about 2 to about 4.5, and most preferably within a range ofabout 2.5 to about 4.

The composition comprises one or more suppressor compounds. In oneembodiment, the one or more suppressor compounds comprise acetylenicalcohol compounds or derivatives thereof as further described herein.The concentration of the suppressor is preferably between about 1 andabout 500 mg/L, and more preferably between about 5 and about 200 mg/L,more preferably between about 1 mg/L and about 70 mg/L, and mostpreferably between about 20 and about 50 mg/L.

The composition may also optionally comprise one or more uniformityenhancing compounds which preferably comprise aminic polyol compounds orderivatives thereof. A preferred uniformity enhancer is ethoxylated,propoxylated triisopropanolamine. In one embodiment, the uniformityenhancer has a molecular weight of about 5000 g/mol. Other preferreduniformity enhancing compounds include ethoxylated, propoxylatedethylene diamine, ethoxylated, propoxylated diethylene triamine andethoxylated, propoxylated triethylenetetramine. If used, theconcentration of the uniformity enhancer is preferably between about 10and about 4000 mg/L, and more preferably between about 100 and about2000 mg/L, and most preferably between about 250 and about 1000 mg/L.

The composition may also optionally comprise one or more depolarizingcompounds. In one embodiment the one or more depolarizing compoundscomprises terminal unsaturated compounds or derivatives thereof, whichare capable of depolarizing the plating potential. In one embodiment,the depolarizing compound may be selected from the group consisting ofsodium propargyl sulfonate, acetylenedicarboxylic acid, acrylic acid,propiolic acid, vinyl phosphonate, and mixtures thereof. One preferreddepolarizing compound is sodium propargyl sulfonate. If used, theconcentration of the depolarizing compound is preferably between about0.1 and about 5000 mg/L, and more preferably between about 10 and about1000 mg/L, and most preferably between about 100 and about 500 mg/L.

In one embodiment, the cobalt electrolyte composition is essentiallyfree of chloride ions, meaning that the chloride content is less thanabout 1 ppm, more preferably less than 0.1 ppm.

The electroplating composition is also preferably free of any functionalconcentration of reducing agents effective to reduce cobaltous ions(Co²⁺) to metallic cobalt (Co⁰). By “functional concentration” what ismeant is any concentration of a reducing agent that is either effectiveto reduce cobaltous ions in the absence of electrolytic current or isactivated by an electrolytic field to react with cobaltous ions.

In addition, the electroplating composition is preferably at leastessentially free of dispersed particles, meaning that there are no orvirtually none macroscopic particulate solids in the solution that aredispersed and would negatively interfere with the metal electroplatingprocess.

In another preferred embodiment, the cobalt composition also optionally,but preferably, comprises one or more levelers, one or more acceleratorsand/or one or more wetting agents. In other preferred embodiments thecobalt composition does not contain and is preferably at leastsubstantially free of an accelerator or a depolarizes. In otherpreferred embodiments, the cobalt composition does not contain and ispreferably at least substantially free of a leveler.

When divalent sulfur compounds are excluded from the plating bath, thesulfur content of the cobalt deposit is lowered, providing beneficialeffects in chemical mechanical polishing and circuit performance.

The electrolytic composition is substantially free of divalent sulfurcompounds if the concentration of divalent sulfur in the platingsolution is not greater than 1 mg/l. Preferably, the concentration ofcompounds containing divalent sulfur atoms is not greater than 0.1 mg/l.Still more preferably, the concentration of divalent sulfur atoms isbelow the detection level using analytical techniques common to thoseskilled in the art of metal plating.

To reduce internal stress in the cobalt deposit, the electrolyticcomposition can include a stress reducer such as saccharin. When used,saccharin is present in the electrolytic composition in a concentrationbetween about 10 and about 300 ppm, more preferably between about 100and about 200 ppm.

When the plating bath contains a suppressor as described herein andoptionally a uniformity enhancer, the superfilling process proceedssatisfactorily without the need for an accelerator. The suppressors inthe current invention help drive current into the features to makebottom-up filling efficient and the uniformity enhancing additives helpimprove deposit uniformity. The composition is substantially free ofreducing agents that reduce Co²⁺ to Co⁰, divalent sulfur, copper ions,nickel ions and iron ions.

It has also been found that certain depolarizing compounds can functionin conjunction with the suppressor compounds as described herein. Thesecompounds depolarize the plating potential to efficiently plateinterconnect features.

In one embodiment, the suppressor is an acetylenic suppressor. Theacetylenic suppressor preferably comprises a reaction product of analkoxylated propargyl alcohol or propargyl alcohol with a secondcomponent. Examples of suitable acetylenic suppressors include, but arenot limited to, reaction products of alkoxylated propargyl alcohol orpropargyl alcohol with glycidol, propylene oxide, glycidol and propyleneoxide, or propylene glycol and glycidol. In one embodiment, thealkoxylated propargyl alcohol is ethoxylated propargyl alcohol. However,it is also contemplated that other alkoxylated propargyl alcohols wouldalso be used in the compositions described herein. Examples ofinitiators and reacting species for preparing acetylenic suppressors inaccordance with the present invention are shown below in Table 1.

TABLE 1 Initiators and reacting species for use in preparing acetylenicsuppressors Initiators

Reacting species

Preferred Products

In one embodiment, x and y are between 0 and 20, more preferably between0 and 10. In another preferred embodiment, one of x or y is at least 1.Examples of preferred ratios included, but are not limited to:

x is 0, y is 1 to 3;

y is o and x is 1 to 7; and

x is 1-4 and y is 1-4.

Other ratios of x and y would also be known to those skilled in the artand are usable in the present invention.

Table 2 depicts several examples of preferred acetylenic suppressors inaccordance with the present invention and as formulated using theinitiators and reacting species described herein.

TABLE 2 Preferred specific acetylenic suppressors Ethoxylated propargylalcohol + glycidol (Compound 1)

Ethoxylated propargyl alcohol + propylene oxide (Compound 2)

Ethoxylated propargyl alcohol + glycidol + propylene oxide (Compound 3)

ethoxylated propargyl alcohol + propylene oxide + glycidol (Compound 4)

In one embodiment, n is between 0 and 20, more preferably between 0 and10, most preferably between 1 and 7.

The electrolytic composition described herein can be used in a methodfor filling submicron features of a semiconductor base structure Asubmicron electrical interconnect feature has a bottom, sidewalls, andtop opening. The submicron features comprise cavities in the basestructure that are superfilled by rapid bottom-up deposition of cobalt.A metallizing substrate comprising a seminal conductive layer is formedon the internal surfaces of the submicron features, e.g., by physicalvapor deposition of metal seed layer, preferably a cobalt metal seedlayer, or deposition of a thin conductive polymer layer. The metallizingsubstrate is applied to the bottom and sidewall, and typically to thefield surrounding the feature. The metallizing substrate within thefeature is contacted with the electrolytic composition and current issupplied to the electrolytic composition to cause electrodeposition ofcobalt that fills the submicron features. By co-action of thesuppressor, optional uniformity enhancer, and optional depolarizingcompound, a vertical polarization gradient is formed in the feature inwhich filling will occur by bottom up deposition at a rate of growth inthe vertical direction which is greater than a rate of growth in thehorizontal direction, yielding a cobalt interconnect that issubstantially free of voids and other defects.

Bottom-up fill means that deposit grows up from the bottom of features,like a trench. Bottom-up fill can be classified as V or U shaped. AV-shape bottom-up has a pointer bottom, and a U-shape bottom-up has moreleveled bottom. U-shape bottom-up filling is preferred, as V-shapebottom-up filling can generate seams. Conformal fill means that adeposit grows from sidewalls and bottom to the center of features. Themost challenging features usually have very large aspect ratios (aspectratio is the ratio of depth over width), and conformal fill typicallymakes a seam at the center of such features. A seam can be vague orclear depending on fill mechanism. However, after annealing, any seamcan make seam voids or center voids.

What is meant by “substantially void free” is that at least 95% of theplated features or apertures are void-free. Preferably, at least 98% ofthe plated features or apertures are void-free, most preferably all ofthe plated features or apertures are void-free.

What is meant by “substantially seam-free” is that is that at least 95%of the plated features or apertures are seam-free. Preferably, at least98% of the plated features or apertures are seam-free, most preferablyall of the plated features or apertures are seam-free.

To implement the electrodeposition method, an electrolytic circuit isformed comprising the metallizing substrate, an anode, the aqueouselectrolytic composition, and a power source having a positive terminalin electrically conductive communication with the anode and a negativeterminal in electrically conductive communication with the metallizingsubstrate. Preferably, the metallizing substrate is immersed in theelectrolytic composition. An electrolytic current is delivered from thepower source to the electrolytic composition in the circuit, therebydepositing cobalt on the metallizing substrate.

The electrodeposition process is preferably conducted at a bathtemperature in the range of about 5° C. to about 80° C., more preferablybetween about 20° C. and about 50° C., most preferably at about roomtemperature, and a current density in the range between about 0.01 andabout 20 mA/cm², preferably between about 0.3 and about 10 mA/cm².Optionally, the current may be pulsed, which can provide improvement inthe uniformity of the deposit. On/off pulses and reverse pulses can beused.

Typically, the electroplating bath is agitated during use. Any suitableagitation method may be used with the process described herein includingsparging with air or inert gas, workpiece agitation, impingement, or thelike. In addition, the workpiece may be rotated in the electroplatingsolution. Alternatively, instead of immersing the workpiece into theelectroplating solution, the workpiece maybe contacted with theelectroplating solution by pumping, spraying or other means known tothose skilled in the art.

The present invention also relates generally to a screening method forevaluating the suitability of bath additives for cobalt electroplatingcompositions.

As discussed above, rough or uneven surfaces can cause defects fromchemical mechanical polishing (CMP), so it is critical to have a smoothtopology for overburden. In addition, rough or uneven surfaces can beespecially problematic due to the difference in overburden (OB)thickness between feature and non-feature areas, which is normallyexpected due to momentum plating. This difference in overburdenthickness can be further enhanced depending in part on the density offeatures.

Thus, it would be desirable to have a reliable screening method forevaluating potential cobalt electroplating compositions and additivescontained therein to determine their suitability in producing a cobaltdeposit with minimal change in overburden thickness between feature andnon-feature areas of a substrate.

As described herein, the inventors of the present invention havesurprisingly discovered that cyclic voltammetry can be used to evaluatecobalt electroplating compositions containing various bath additives.The present invention contemplates an electrochemical screening methodusing cyclic voltammetry to evaluate cobalt electroplating compositions.

In the cyclic voltammetric scan, the forward scan represents non-featureareas of plating and the backward scan represents feature areas of theplating. The potential difference between the forward scan and backwardscan is referred to herein as the “hysteresis loop.” A larger hysteresisloop means a larger potential difference and vice versa. In addition,the size of the hysteresis loop can vary with the current density.

The inventors of the present invention have discovered that a cobaltelectroplating composition that exhibits a small hysteresis loop at highcurrent density has a positive impact on overburden topography. Thehysteresis loop at the high current density for OB, which is generallyat least about 5 mA/cm², can have a negative impact on OB topology. Theinventors of the present invention have discovered that a clearcorrelation exists between the hysteresis loop at high current densityand OB topology. It was found that a larger hysteresis loop can lead toa larger difference in OB thickness between feature and non-featureareas, which can cause defects in CMP and thus is not preferred for CMP.

In one embodiment, the hysteresis loop at high current density is lessthan 50 mV, preferably less than 40 mV, more preferably less than 30 mV,more preferably less than 20 mV, most preferably less than 10 mV. Byhigh current density what is meant is a current density that is in therange of at least about 5 mA/cm², preferably in the range of about 5 toabout 10 mA/cm², more preferably in the range of about 7.5 to about 8.5mA/cm² and most preferably at about 8 mA/cm². In one embodiment, it isdesirable that the hysteresis loop at a current density of about 8mA/cm² be less than about 50 mV, preferably, less than about 40 mV, morepreferably less than about 30 mV.

It has also been observed that most of the chemistry of good gap fillperformance has a hysteresis loop at low current density, although thereis no clear correlation that a larger hysteresis loop can make gap fillbetter.

It is highly desirable that the change in overburden thickness betweenfeature and non-feature areas. In one embodiment, the change inoverburden thickness is controlled to be less than 50 nm, morepreferably less than 40 nm, or less than 35 nm, or less than 30 nm, orless than 20 nm, or less than 10 nm.

TABLE 3 Hysteresis Loop and Overburden Thickness Difference OB ThicknessDifference Hysteresis Loop, mV between feature and Chemistry At 2 mA/cm²At 8 mA/cm² non-feature area, nm Comparative 82 55 83 Example 1 Example2 81 33 52 Example 3 88 41 41Plating Experimental:Temperature—room temperatureWaveform:OCP followed by 0.5 to 2 mA/cm² ramp current for the amount time neededto complete via fill; then 8 mA/cm² for the amount of time required toobtain desired overburden thickness.

Comparative Example 1

An aqueous cobalt electroplating bath was prepared containing 2.95 g/LCo²⁺ ions, 30 g/L boric acid, and sulfuric acid to adjust pH to 2.75.Ethoxylated propargyl alcohol (40 mg/L) and an aminic polyol with MWaround 5000 (50 mg/L) are also added to the composition.

A substrate with submicron features is placed into the bath andelectroplated from 0.5 to 8 mA/cm² to provide seam-free fill with overburden. A cyclic voltammetric scan was performed using the waveformdescribed above to measure hysteresis loop.

The OB thickness difference (uniformity) and CV hysteresis loopmeasurements are described in Table 3 and that film impurities aresummarized in Table 4.

Example 2

An aqueous cobalt electroplating bath was prepared containing 2.95 g/LCo²⁺ ions, 30 g/L boric acid, and sulfuric acid to adjust pH to 2.75.Compound I (See Table 2, x=3.6), was added to the bath at aconcentration of 240 mg/L.

A substrate with submicron features is placed into the bath andelectroplated from 0.5 to 8 mA/cm² to provide seam-free fill with overburden. A cyclic voltammetric scan was performed using the waveformdescribed above to measure the hysteresis loop.

The OB thickness difference (uniformity) and CV hysteresis loopmeasurements are described in Table 3 and that film impurities aresummarized in Table 4.

Example 3

An aqueous cobalt electroplating bath was prepared containing 2.95 g/LCo²⁺ ions, 30 g/L boric acid, and sulfuric acid to adjust pH to 2.75.Compound III (See Table 2, x=3.6 and y=1), was added to the bath at aconcentration of 240 mg/L.

A substrate with submicron features is placed into the bath andelectroplated from 0.5 to 8 mA/cm² to provide seam-free fill with overburden. A cyclic voltammetric scan was performed using the waveformdescribed above to measure the hysteresis loop.

The OB thickness difference (uniformity) and CV hysteresis loopmeasurements are described in Table 3 and that film impurities aresummarized in Table 4.

FIG. 1 depicts a Cyclic Voltammetry (CV) scan from +10 mV vs OCP to −2Vvs RE at 2 mV/s with 100 rpm for Comparative Example 1, Example 2 andExample 3.

TABLE 4 Film Impurities in Examples Impurities, PPMW Chemistry C S Cl NTotal VMS Only 28.5 0.5 0.2 0.2 29.4 Comparative Example 1 232.4 3.2 0.10.5 236.2 Example 2 275.2 0.5 0.3 0.2 276.2 Example 3 87.9 0.3 0.3 0.388.8

As shown in FIG. 1, hysteresis loop is the potential difference betweenforward and backward scan at the same current density. At low currentdensity, hysteresis loop represents gap fill capability; and at highcurrent density, hysteresis loop represents the performance of OBISO/Dense uniformity. The hysteresis loop of curves in FIG. 1 issummarized in Table 3. It is noted that 2 mA/cm² represents low currentdensity; and 8 mA/cm² represents high current density.

In addition, in Table 3, the last column summarizes OB thicknessdifference between feature and non-feature area using the internaltesting structure described herein. This internal testing structure isreferred to as “454.” The feature area of the test structure has atrench of 0.1 um opening with 0.1 um spacing, and is considered a densefeature. The non-feature area is the area next to the feature area. TheOB thickness difference can represent OB ISO/Dense uniformityperformance. The smaller difference means better OB ISO/Dense uniformityperformance. In Table 3, it is clear that Comparative Example 1 exhibitsa large hysteresis loop at 8 mA/cm² has larger OB thickness differencethan Examples 2 and 3 which have small hysteresis loop at 8 mA/cm².

It is believed that the hysteresis loop at low current density canrepresent gap fill capability. In other words, if two chemistry havingsimilar hysteresis loop at low current density should be able to deliversimilar gap fill performance. In one embodiment, the plated cobaltdeposit exhibits a potentiodynamically measured hysteresis loop in acyclic voltammetry scan at a current density in the range of about 1 toless than 5 mA/cm², more preferably in the range of about 1.5 to about2.5 mA/cm², of less than about 90 mV. In Table 3, it is clear thatComparative Example 1, Example 2, and Example 3 all have similarhysteresis loop at low current density, 2 mA/cm², and it is observedthat they have similar gap fill performance in TEM or STEM.

The idea of hysteresis loop theory is to have a chemistry which can havelarge hysteresis loop at low current density and small hysteresis loopat high current density to achieve both good gap fill and OB ISO/Denseuniformity at the same time

Table 4 summarizes SIMS (secondary ion mass spectrometry) data anddemonstrates the impurity level in deposit. The goal of this study is toachieve as little impurity in deposit as possible. Ideally the same asVMS only, or less. At the unique molecular structure of Example 2, itcan have much less impurity than the other chemistry, and it can stilldeliver proper gap fill performance.

Based thereon, it can be seen that a smaller hysteresis loop at 8 mA/cm²where OB is plated can lead to less difference in OB thickness betweenfeature and non-feature areas.

This observation can also be supported by theory. Momentum plating isexpected on feature areas. This means that the feature area has a fasterdeposit growth rate than non-feature areas. The adsorbed acetylenicsuppressor on feature area surfaces is also expected to be less thannon-feature areas due to more fresh deposit surface. At the forward scanpart, the potential is canned from low to high and at the backward scanpart, the potential is scanned from high to low. So, more fresh depositsurface is expected to be formed during the forward scan. As a result,in the CV scan, the forward scan represents non-feature area plating andthe backward scan represents feature area plating.

In the CV curve, at the same current density, when the forward scan ismore cathodic than the backward scan, this is an indication that ittakes more energy to grow on non-feature areas than on feature areas. Inother words, the growth of the deposit on feature areas is enhanced,which can cause a difference in deposit thickness between feature andnon-feature areas. This is the fundamental reason why minimizing thehysteresis loop at the current density where the OB is plated iscritical to reduce the difference in deposit thickness between featureand non-feature areas.

Thus it can be seen that the invention described herein provides animproved method and composition for cobalt-based metallization ofinterconnect features in semiconductor substrates. In addition, theinvention described herein provides a screening method for evaluatingand identifying suitable bath additives to achieve a desired result.

Finally, it should also be understood that the following claims areintended to cover all of the generic and specific features of theinvention described herein and all statements of the scope of theinvention that as a matter of language might fall there between.

What is claimed is:
 1. A method of electrodepositing cobalt onto asubstrate comprising recessed features and non-feature areas, whereinthe cobalt deposit exhibits a thickness difference across the surface ofthe substrate of less than about 200 nm the method comprising: a)receiving the substrate in an electroplating chamber; b) immersing thesubstrate into a cobalt electrolyte, the cobalt electrolyte comprising:(1) a source of cobalt ions; (2) boric acid; (3) a pH adjuster; and (4)an organic additive, wherein the organic additive comprises asuppressor; and c) electroplating cobalt into the features and onto thenon-feature areas for a period of time and under conditions to achieve alevel, seam-free, bottom-up fill in the recessed features and plating onthe non-feature areas; wherein the cobalt deposit exhibits apotentiodynamically measured hysteresis loop in a cyclic voltammetryscan at a current density in a range of 5 to about 10 mA/cm² of lessthan about 50 mV.
 2. The method according to claim 1, wherein thesubstrate has a cobalt seed layer disposed thereon and the cobalt iselectrodeposited onto the cobalt seed layer.
 3. The method according toclaim 1, wherein the electroplated cobalt deposit exhibits a thicknessdifference of less than about 100 nm.
 4. The method according to claim3, wherein the electroplated cobalt deposit exhibits a thicknessdifference of less than about 75 nm.
 5. The method according to claim 4,wherein the electroplated cobalt deposit exhibits a thickness differenceof less than about 50 nm.
 6. The method according to claim 1, whereinthe potentiodynamically measured hysteresis loop in the cyclicvoltammetry scan at the current density in the range of 5 to about 10mA/cm² is less than about 30 mV.
 7. The method according to claim 6,wherein, the potentiodynamically measured hysteresis loop in the cyclicvoltammetry scan at the current density in the range of 5 to about 10mA/cm² is less than about 20 mV.
 8. The method according to claim 7,wherein the potentiodynamically measured hysteresis loop in the cyclicvoltammetry scan at the current density in the range of 5 to about 10mA/cm² is less than about 10 mV.
 9. The method according to claim 1,wherein the current density at which the hysteresis loop is measured isin a range of about 7.5 to about 8.5 mA/cm².
 10. The method according toclaim 9, wherein the current density at which the hysteresis loop ismeasured is about 8 mA/cm².
 11. The method according to claim 1, whereinthe plated cobalt deposit also exhibits a potentiodynamically measuredhysteresis loop in a cyclic voltammetry scan at a current density in arange of about 1 to less than 5 mA/cm² of less than about 90 mV.
 12. Themethod according to claim 11, wherein the current density at which thehysteresis loop is measured is in a range of about 1.5 to about 2.5mA/cm².
 13. The method according to claim 1, wherein in the cyclicvoltammetry scan, a forward scan represents non-feature area plating anda backward scan represents feature area plating of the cobalt deposit.14. The method according to claim 1, wherein the cobalt electrolyte isat least substantially free of an accelerator.
 15. The method accordingto claim 1, wherein the cobalt electrolyte is at least substantiallyfree of a depolarizer.
 16. The method according to claim 1, wherein theorganic additive comprises an acetylenic suppressor.
 17. The methodaccording to claim 16, wherein the acetylenic suppressor comprises areaction products of an alkoxylated propargyl alcohol or propargylalcohol with glycidol, propylene oxide, glycidol and propylene oxide, orpropylene glycol and glycidol.
 18. The method according to claim 17,wherein the alkoxylated propargyl alcohol is ethoxylated, propargylalcohol.
 19. The method according to claim 1, wherein the recessedfeatures are damascene features.
 20. A method of electrodepositingcobalt onto a substrate comprising recessed features and non-featureareas, wherein the cobalt deposit exhibits a thickness difference acrossthe surface of the substrate of less than about 200 nm the methodcomprising: a) receiving the substrate in an electroplating chamber; b)immersing the substrate into a cobalt electrolyte, the cobaltelectrolyte comprising: (1) a source of cobalt ions; (2) boric acid; (3)a pH adjuster; and (4) an organic additive, wherein the organic additivecomprises a suppressor; and c) electroplating cobalt into the featuresand onto the non-feature areas for a period of time and under conditionsto achieve a level, seam-free, bottom-up fill in the recessed featuresand plating on the non-feature areas; wherein the plated cobalt depositexhibits a potentiodynamically measured hysteresis loop in a cyclicvoltammetry scan at a current density in a range of about 1 to less than5 mA/cm² of less than about 90 mV.
 21. The method according to claim 20,wherein the cobalt deposit also exhibits a potentiodynamically measuredhysteresis loop in a cyclic voltammetry scan at a current density in arange of 5 to about 10 mA/cm² of less than about 50 mV.